The Mail app will verify the account and take you back to set additional Outgoing Server Info, add 465 in the Port field and tick Use SSL. Make sure Password is selected in the Authentication dropdown.

You've completed setting up e-mail on Yosemite's Mail app. You can send
and receive your email on your email client using a secure connection.

Troubleshooting/verifying settings for Mac Mail on Yosemite

With Apple Mail open, click Mail from the top left of the screen and then select Preferences. Select your account and verify that the proper data is filled in.

Open the SMTP panel to verify Outgoing server data and port values. These should be:

Port: 465 / SSL On

Username: your email address

Password: your password

Check Advanced for port values on the incoming server. This should be port 993 for IMAP and 995 for POP, with SSL On.